1. Field of the Invention
The present invention relates to a fixing apparatus which is mounted on an image forming apparatus to form an image on a transfer material using an electrophotographic process, a copying machine, a printer, or the like, and which fixes a developer on the transfer material onto the transfer material.
2. Description of the Related Art
It has been known that in a copying machine or a printer using an electrophotographic process, a toner image formed on a photosensitive drum is transferred onto a transfer material, and thereafter the toner image molten in a fixing apparatus including a heating roller and a pressurizing roller is fixed onto the transfer material.
In recent years, as a method of heating the heating roller, an example has been known in which a heat-resistant film material having a thin metal layer (conductive film) is formed in an endless belt form or a cylindrical shape (roller) and is brought into contact with a member to be fixed using induction heating. Accordingly, as compared with a heating method using a lamp or the like, response to a temperature change of the heating roller increases, temperature instantly rises, and warming-up time can be shortened.
Moreover, an example has been known in which a plurality of heating portions (coils) using the induction heating are arranged in a longitudinal direction of the heating roller to heat a predetermined region of the heating roller selected in accordance with a size or the like of a fixing sheet.
At this time, a method is known in which surface temperature is detected using a detection element brought into contact with the surface of the heating roller to control the temperature of the heating roller.
However, the response of temperature detection of the contact temperature detection element is lower than that (heating response) to a temperature rise of the heating roller heated by the induction heating, and a time lag sometimes occurs. There is a problem that the temperature of the heating roller rises above a fixing temperature and overshoot occurs.
Moreover, there is a problem that a correct temperature of the heating roller cannot be detected by a shift between the response of the detection of the contact temperature detection element and the heating response of the heating roller. Accordingly, when a plurality of coils are arranged in the longitudinal direction of the heating roller, there is a problem that a temperature unevenness is caused in a predetermined region of the heating roller heated by the different coils. This temperature unevenness causes a high-temperature offset or a low-temperature offset in the longitudinal direction of the heating roller, and causes a problem that a defect is caused in the image on the fixing sheet in a main scanning line direction.